In many patients who are blinded by degenerative conditions, the photoreceptors of the retina may no longer function normally. For many of these patients, however, the retinal ganglion cells can continue to function and provide a signal pathway through the central nervous system to the brain.
A typical eye 10 and the relative location of its components is schematically illustrated in FIGS. 1 and 2. The eye 10 is a generally circular globe filled with aqueous humour 12, a clear liquid that is similar to water. The eye 10 also includes 20 cornea 14, which is a transparent structure that admits light into the eye 10. The amount of light passing into the eye 10 is controlled by an iris 16, a muscle that moves to allow or block light from passing through a lens 20 behind the iris 16 to the interior of the eye 10. The lens 20 focuses light passing therethrough onto the retina 22. The retina 22 forms the interior surface of the eye opposite the lens. The output of the retina 22 is carried by retinal ganglion cells 30 that transmit action potentials to the brain via the optic nerve 24.
In the eye 10, the retina 22 is a multilayered tissue that includes a layer of rods 26 and cones 28 which are the photoreceptors that detect the light falling thereon and help to convert the image projected on the retina 22 into electrical signals that can be interpreted by the brain as sight. The rods 26 provide vision in dim light and do not respond to bright light. Cones 28 on the other hand, do not respond to dim light, but provide color and fine detail vision. An inner nuclear layer separates the rods 26 and cones 28 from the ganglion cells 30 and includes amacrine cells 32, bipolar cells 34 and horizontal cells 36.
A common structural feature of ganglion cells 30 is an approximately 90° bend 40 in the axon as the axon leaves the ganglion cell layer 42 and enters the nerve fiber layer 44. The axons of the retinal ganglion cells 30 pass across the surface of the retina 22 and collect before exiting to form the optic nerve 24. In a healthy eye, light detected by the rods 26 and cones 28 generate signals that are transmitted via the retinal ganglion cells 30 and the optic nerve 24 to the brain, which perceives the visual image.